The surfaces of plastic containers, cartons, vinyl bags, boxes and envelopes are usually subjected to flexographic printing. As flexographic plates used in this flexographic printing, those formed by exposure, to light, of a composition for photosensitive flexographic plates which comprises an elastomer, a polymerizable ethylenically unsaturated monomer and a photopolymerizable initiator, are frequently used.
The composition for photosensitive flexographic plates is usually formed into a sheet and supplied as a multilayer sheet having a plastic sheet as a supporting body arranged on one side and a protective film arranged on the other side. By exposing the multilayer sheet to light at the side of the supporting body, the layer of the composition for photosensitive flexographic plates is cured to specific thickness. Then, the protective film is removed and a negative film is stuck to the surface of the composition layer, and the negative film is irradiated with light. The light-permeated part of the composition for photosensitive flexographic plates is cured, and an uncured part is removed by an organic solvent or an aqueous solvent, whereby a flexographic plate having an embossed structure is formed.
For the purpose of facilitating formation of an accurate embossed surface, it is desired that when the composition for photographic flexographic plates is formed into a sheet, a smooth surface is formed, and when the sheet-shaped composition for photographic flexographic plates is stored, the thickness of the sheet is hardly changed (due to excellent antiflowing properties) regardless of its own weight or a loading on the sheet.
For the purpose of removing the protective film without roughing the surface, the composition for photographic flexographic plates is required to be hardly sticky.
For high-quality printing, the composition for photographic flexographic plates is required to be excellent in thin-line reproducibility in a negative film.
Conventionally, JP-B 7-72795, for example, discloses a composition for photosensitive flexographic plates, which comprises a linear aromatic vinyl/conjugated diene/aromatic vinyl block copolymer, an ethylenically unsaturated monomer and a photopolymerization initiator, said block copolymer being obtained by using, as a coupling agent, an ester compound made from a monovalent aliphatic carboxylic acid and a monovalent alcohol. Such composition gives a flexographic plate excellent in antiflowing properties and thin-line reproducibility, but there is the case where the surface of the sheet is poor in smoothness and becomes easily sticky.
JP-A 10-288838 discloses a composition for photosensitive flexographic plates, which comprises a hydrogenated block copolymer obtained by selectively hydrogenating a conjugated diene unit in the styrene/conjugated diene/styrene block copolymer, an ethylenically unsaturated monomer and a photopolymerization initiator. Such composition gives a flexographic plate less apt to occur troubles caused by sticking and excellent in thin-line reproducibility, but there is the case where the surface of the sheet is inferior in smoothness and antiflowing properties.
Japanese Patent Application National Publication (Laid-Open) No. 2002-519465 discloses a composition for photosensitive flexographic plates, which comprises a block copolymer composition, an ethylenically unsaturated monomer and a photopolymerization initiator, wherein the block copolymer composition comprises an aromatic vinyl/conjugated diene/aromatic vinyl block copolymer containing 5 to 25% by weight aromatic vinyl monomer unit, and an aromatic vinyl/conjugated diene block copolymer having a weight-average molecular weight of 1,000 to 35,000. Such composition gives a flexographic plate excellent in thin-line reproducibility, but there is the case where the surface of the sheet is inferior in smoothness and antiflowing properties, and troubles caused by sticking occur.